Poor understanding of accumulation leads to serious errors in reasoning about climate change (see charts, left, and on page 533). Sterman and Booth Sweeney (14) gave 212 graduate students at the Massachusetts Institute of Technology (MIT) a description of the relationships among GHG emissions, atmospheric concentrations, and global mean temperature. The description was excerpted from the IPCC's "Summary for Policymakers" (SPM), a document intended for nonspecialists (4). Participants were then asked to sketch the emissions trajectory required to stabilize atmospheric CO2. To highlight the stock-flow structure, participants were first directed to estimate future net removal of CO2 from the atmosphere (net CO2 taken up by the oceans and biomass), then draw the emissions path needed to stabilize atmospheric CO2 [the SOM (2) provides details].

Knowledge of climatology or calculus is not needed to respond correctly. The dynamics are easily understood using a bathtub analogy in which the water level represents the stock of atmospheric CO2. Like any stock, atmospheric CO2 rises when the inflow to the tub (emissions) exceeds the outflow (net removal), is unchanging when inflow equals outflow, and falls when outflow exceeds inflow. Participants were informed that anthropogenic CO2 emissions are now roughly double net removal, so the tub is filling.

Yet, 84% drew patterns that violated the principles of accumulation. If emissions followed the path in the typical example shown, atmospheric CO2 would continue to rise. Nearly two-thirds of the participants asserted that atmospheric GHGs can stabilize even though emissions continuously exceed removal--analogous to arguing a bathtub continuously filled faster than it drains will never overflow. Most believe that stopping the growth of emissions stops the growth of GHG concentrations. The erroneous belief that stabilizing emissions would quickly stabilize the climate supports wait-and-see policies but violates basic laws of physics.

Training in science does not prevent these errors. Three-fifths of the participants have degrees in science, technology, engineering, or mathematics (STEM); most others were trained in economics. Over 30% hold a prior graduate degree, 70% of these in STEM. These individuals are demographically similar to influential leaders in business, government, and the media, though with more STEM training than most.

It is tempting to respond to these discouraging results by arguing that poor public understanding of climate change is unimportant because policy should be informed by scientific expertise. Many call for a new Manhattan Project to address the challenge (15, 16). The desire for such technical solutions is understandable. In 1939, scientists directly alerted the nation's leaders to developments in atomic physics, then, by focusing enough money and genius in the deserts of New Mexico, created nuclear weapons in just 6 years. Science has arguably never affected geopolitical outcomes more decisively.

But a Manhattan Project cannot solve the climate problem (17). The bomb was developed in secret, with no role for the public. In contrast, reducing GHG emissions requires billions of individuals to cut their carbon footprints by, e.g., buying efficient vehicles, insulating their homes, using public transit, and, crucially, supporting legislation implementing emissions abatement policies. Changes in people's views and votes create the political support elected leaders require to act on the science. Changes in buying behavior create incentives for businesses to transform their products and operations. The public cannot be ignored.